The present invention relates generally to passive methods and apparatus for measuring fissionable materials which emit neutrons and relates more particuarly to passive methods and apparatus for measuring spent fuel assemblies.
Assaying (i.e., measuring the fissionable content of) spent fuel is important for nuclear safeguards in order to prevent unauthorized diversion of the nuclear material, to provide information necessary for criticality control of spent-fuel storage pools, and to provide process control for reprocessing and reactor operation. Measurement techniques for the assay of spent fuel have included passive and active neutron methods and passive gamma-ray methods. In active neutron methods, an external isotopic neutron source is used to interrogate the spent fuel, whereas in passive methods, no external neutron source is used.
When an assay is performed, the goal is to determine the fissile content (i.e., the amounts of uranium and plutonium) in the material being assayed. It is well known in the art that the ratio of Pu/U and fissile content can be correlated to both burnup and reactivity, where burnup is a measure of the number of fissions which occurred in the fuel while the fuel was within the reactor and where reactivity is related to burnup. This is disclosed for example in S. T. Hsue et al., "Nondestructive Assay Methods for Irradiated Nuclear Fuels," Los Alamos Scientific Laboratory Report LA-6923, January 1978. Reactivity depends upon how long the fuel has been in the reactor and not upon cooling time (which is the length of time the fuel has been out of the reactor). Fissile content, reactivity, and burnup can be correlated. Therefore, these quantities can be considered equivalent because the measurement of one quantity can provide a means of determining the other quantities.
Passive gamma-ray measurements and passive neutron measurements have been correlated with burnup and have provided a means of verifying the fissile material inventory of spent fuels. However, passive gamma-ray assay is not sensitive to the interior of the spent fuel assembly and therefore cannot truly verify the integrity of the interior fuel rods. Passive gamma-ray assay makes the assumption that the interior fuel rods are present in the fuel assembly and have not been tampered with.
The technique of passive neutron assay relies on correlations between the neutron emission rate and declared burnup to determine the fissile content of the spent fuel assembly. The neutron emission rate is N passive=M.multidot.S, neutron rate where M is the multiplication of the assembly (which is defined as M.tbd.1/(1-k.sub.eff) where k.sub.eff is the effective multiplication constant of the fuel asembly) and S is the spontaneous fission rate of the isotopes of Pu and Cm and the emission rate due to the (.alpha.,n) reactions. For pressurized-water reactor (i.e., PWR) fuel assemblies, good correlation between the U and Pu content and the neutron rate has been observed, although a substantial fraction of the neutron emission rate is due to Cm isotopes. This good correlation is due primarily to the fact that the Cm production rate is a uniform function of burnup. However, good correlations do not always exist for boiling-water reactor (BWR) fuel assemblies because (as has recently been shown by T. Yokoyama et al. in "Measurement and Analysis of Neutron Emission Rate for Irradiated BWR Fuel," Journal of Nuclear Science and Technology, 18, pp. 249-260 (April 1981)) the relationship between the neutron emission rate and burnup for these assemblies is double-valued (rather than a single-valued functional relationship). The transuranic production chain depends on the thermal-to-epithermal neutron ratio in the irradiation environment; and in BWR reactors this ratio depends upon the void fraction. For the same burnup, the upper portion of the fuel assembly has more transuranium nuclides than the lower portion because the increased void fraction in the upper portion of the fuel assembly causes the neutron energy spectrum to be harder (i.e., have higher energy) than in the bottom portion of the fuel assembly.
Active assay systems (wherein an external isotopic neutron interrogating source or external fissile material is used) have long been considered to give the best assay results. A determination of the amount of fissionable material and a determination of the reactivity can both be made by using active assay techniques. Here, an external neutron source or a neutron source and external fissile material induces the fissions in the U and Pu isotopes. The fission neutrons are detected and correlated to either burnup, the fissile content, or reactivity. The interrogating sources can be either isotopic sources (for example, .sup.252 Cf, .sup.124 Sb-Be, AmLi, etc.) or can be accelerator sources (for example, sealed-tube neutron generators). Systems based on prompt and delayed neutron counting using either .sup.252 Cf, .sup.124 Sb-Be, or neutron generators have been designed and tested; however, these systems require strong sources (e.g., 3Ci .sup.252 Cf and 800Ci .sup.124 Sb-Be) so that the induced signals can be measured in the presence of strong passive neutron emission rates (which are the noise in the system). For high burnup and freshly discharged reactor fuel, the strength of the interrogating source needed to overcome the passive neutron rate can be prohibitively large. Furthermore, when an active system is used to assay, either the fuel material being assayed must be scanned or multiple isotopic sources must be used. Additionally, with active assay systems the measurement geometry is limited because the neutron source is usually a point source.
Therefore, despite the assay systems which have been available in the prior art, a need has existed until now for a method and apparatus which has the measurement capability of an active system for measuring both reactivity and content of fissionable material but which does not require scanning nor use of an external isotopic neutron source or external fissile material and which in particular does not require use of large isotopic sources.